1. Field of the Invention
The subject invention is generally related to high temperature ceramic particulate filters and, more particularly, to candle filters used in coal combustion systems and a method for their manufacture.
2. Description of the Prior Art
A number of high temperature filtration systems for coal combustion are currently being developed by a number of companies. The economic viability, efficiency, and durability of these systems are largely dependent on the delivery of particulate-free hot gas to the power turbine. A typical filtration system will operate up to approximately 1 megapascal (MPa), which is equivalent to 10 atmospheres or 150 pounds per square inch (psi), and 1000.degree. C. (1850.degree. F.). The filters are subjected to a wide range of stress-inducing forces including vibrations and thermal shock resulting from pulse cleaning. In addition, the filters are subject to corrosion due to the alkali and steam present in the combustion gases. Low level gas phase contaminants such as sulfur dioxide, hydrogen sulfide, and calcium oxide are also sources of corrosion.
The drawing FIGURE shows the flow of gas in a candle filter module. Hot gas from a coal furnace is directed across a plurality of candle filters which hang from a tube sheet. The candle filters are hollow cylindrical members with closed bottom ends and open top ends and are typically hung from the tube sheet by a flange which protrudes from the top end of the candle filter. A typical candle filter is 2-5 inches in outer diameter, 1-3 inches in inner diameter, and 3-4 feet in length. The candle filters are porous and are designed such that particles present in the raw gas are trapped on the outer surface of the candle filters while the gas is permitted to pass through. The clean gas passes upwards through the hollow section of the candle filter and out to a turbine or other element. Over time, a dust cake builds up on the outer surface of the candle filters. To remove the dust cake, a pulse of air is delivered down through the hollow portion of the candle filter which exerts a reverse gas pressure on the outside of the candle that breaks the dust cake from the surface. Fragmented pieces of the dust cake fall into a collection container under gravitational forces.
Prior art candle filters are typically fabricated by bonding silicon carbide (SiC), alumina-silica, mullite, or cordierite grains with a clay binder. Aggregate size (grain) affects the porosity and permeability. It is, of course, the dust cake that builds up on the filtering surface (outside wall) that constitutes the filter most of the time, and is responsible for the filtering efficiency. Some candles incorporate a layer of ceramic fibers or fine grains on the filtering surface (outside wall) of the candle to prevent deep penetration of particles into the filter. Prior art filters have not proved to be durable enough for continuous high temperature use. The clay binder used to bond the ceramic grains eventually forms a brittle glass matrix that is easily damaged by the thermal shock encountered during pulse cleaning. These filters have also tended to be subject to extensive corrosion damage from the alkali and steam which is present in direct coal fired turbine (DCFT) environments.